Donald Trump has accused FBI and US Justice Department leaders of 'politicising' investigations in favour of Democrats.
It comes amid the ongoing tension in Washington DC over a controversial and secret memo, which is expected to be released publicly shortly by Republican lawmakers.
According to The Washington Post, the memo - which was prepared by the office of House Intelligence Committee chairman Devin Nunes - is said to raise concerns about the surveillance of former Trump campaign adviser Carter Page.
Republican members of the committee have voted to publicly release the memo, with the White House having the final say on whether the release should be blocked on national security grounds.
The FBI earlier this week took the rare step of publicly commenting on the document, saying the agency had "grave concerns about material omissions of fact that fundamentally impact the memo’s accuracy".
Democrats, meanwhile, have accused Republicans of using the memo as part of partisan efforts to "discredit the investigations into the Trump-Russia scandal".
The opposition party's leaders in both the House and Senate have called for Nunes to be removed from his committee role over claims he had altered parts of the memo before sending it to the White House.
We have to be able to trust that the House Intel Chairman will put long-term national security before short-term politics. @DevinNunes has proven he can’t.#RemoveNunes, @SpeakerRyan. pic.twitter.com/rgijiNpawY
— Nancy Pelosi (@NancyPelosi) February 1, 2018
Speaker Ryan should immediately remove Rep. Nunes as chair of House Intel committee and rein in the House GOP. #RemoveNunes 2/4
— Chuck Schumer (@SenSchumer) February 1, 2018
Without referencing the memo, President Trump - who, according to US media reports, is not planning to block the memo's release - took to Twitter early on Friday morning local time to allege politicisation by officials and investigators:
The top Leadership and Investigators of the FBI and the Justice Department have politicized the sacred investigative process in favor of Democrats and against Republicans - something which would have been unthinkable just a short time ago. Rank & File are great people!
— Donald J. Trump (@realDonaldTrump) February 2, 2018
He later quoted a post from Conservative activist Tom Fitton, saying: "You had Hillary Clinton and the Democratic Party try to hide the fact that they gave money to GPS Fusion to create a Dossier which was used by their allies in the Obama Administration to convince a Court misleadingly, by all accounts, to spy on the Trump Team."
While it has not yet been made public, the memo is believed to allege that the FBI and Justice Department overstepped their authority to obtain a Foreign Intelligence Surveillance Court (FISA) warrant.
It reportedly suggests the agencies relied on claims within a controversial and unverified dossier prepared on Donald Trump and Russia.
That dossier - which contains various as yet unverified allegations - was prepared by former British spy Christopher Steele.
It has subsequently emerged the document was initially produced as part of opposition research during the 2016 election campaign.